The United States and the European Union have been making efforts to store carbon dioxide underground as an important measure for reducing greenhouse gases, which include carbon dioxide, methane, and hydrochlorofluorocarbon, with carbon dioxide constituting 90% or more of the greenhouse gases. When a large amount of carbon dioxide is injected into a deep aquifer in the form of a gas or supercritical liquid, it tends to solidify (as plume), which leads to low permeability or low diffusivity of the carbon dioxide. Carbon dioxide in the form of a gas or supercritical liquid is lighter than water, and therefore there is a risk that it will leak up to the surface of the ground. The amount of carbon dioxide being generated in the world is extremely large, and therefore there is desired a processing method that efficiently processes carbon dioxide so that said carbon dioxide has good permeability and diffusibility, and that can be fixed in a safe and sure manner underground even in a normal geological environment.
Patent Document 1, “Gas Liquefaction and Sedimentation Device,” discloses that carbon dioxide, which is liquefied under a high pressure, and seawater are alternately pressure-fed deep into the sea. Patent Document 2, “Method for Feeding Liquid Carbon Dioxide into Deep Seawater,” discloses that—utilizing liquefied carbon-dioxide gas, which is heavier than seawater—liquefied gas is released in shallow seawater in the form of liquid bubbles, and fed deep into the sea, maintaining a state of equilibrium. However, both of these documents refer to feeding of the gas into seawater, and do not disclose the permeability or diffusivity of the gas in case the gas is stored under the ground.                [Patent Document 1] Published Japanese Patent Document No. 2000-227085        [Patent Document 2] Published Japanese Patent Document No. H11-228122        